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Senator Rob Portman wrote of how he had "wrestled" with his faith

Influential US Republican Senator Rob Portman has renounced his opposition to gay marriage.

The Ohio senator said he began to change his mind in 2011 after his son, Will, revealed he was gay.

Sen Portman said that his former views on marriage had stemmed from his Methodist faith.

In 1996, as a member of the House of Representatives, he voted for the Defense of Marriage Act, which defines marriage as between a man and a woman.

He also voted in 1999 against allowing gay couples in Washington DC to adopt children.

Senator's surprise

The gay marriage issue has increasingly divided the Republican party, with some members arguing that opposition to same-sex unions contributed to the party's losses nationally in last November's elections.

Ultimately, it came down to my belief that we are all children of GodSenator Rob Portman

Sen Portman, who was considered a potential running mate for Republican White House candidate Mitt Romney, announced his change of heart in interviews with several Ohio newspapers.

"I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn't deny them the opportunity to get married," he wrote in an opinion piece for the Columbus Dispatch.

"That isn't how I've always felt," he wrote. "As a congressman, and more recently as a senator, I opposed marriage for same-sex couples."